This invention relates to so-called compound or hybrid aircraft, that is, those having both a main rotor and a small wing.
Compound helicopters such as the Cheyenne (AH-56A) have a main and tail rotors, but in addition they possess a small fixed wing and a rear propulsion propeller adjacent to the tail or antitorque rotor. The tail gearbox is driven through a drive train coupled to the main rotor drive shaft. Such aircraft are capable of both hover and high speed forward flight, but they require a large vertical lift force in hover, and a large horizontal thrust force at high forward speeds. For a winged compound helicopter which utilizes a pusher propeller for horizontal thrust, the hover and low speed propulsive forces are provided by the helicopter main rotor, with anti-torque/directional control being provided by a smaller tail rotor. In this hover and low speed flight regime both main rotor and tail rotor thrust requirements are high, with the pusher propeller rotating but producing no thrust. In the high speed forward flight regime, part of the required lift is transferred from the main rotor to the wing and directional control is obtained through normal aircraft control surfaces, thus eliminating the need for tail rotor thrust. Forward propulsive force is generated by the pusher propeller.
Generally, during high speed flight a compound helicopter requires full power to the pusher propeller, located at the extremity of the aft fuselage, with no thrust required from the tail rotor. Likewise in low speed and hover flight the tail rotor power requirement is approximately ten percent of total aircraft power with the remaining power being absorbed by the main rotor, which provides the required lift. This is generally true for all sizes of compound helicopters.
In an aerodynamically optimized tail drive propeller/rotor system it can be found that the propeller normally operates at speeds of approximately fifteen percent higher than the tail rotor. Thus in the high speed crusing flight regime almost the entire engine output is fed to the tail gearbox to drive the propulsion propeller, the main rotor absorbing only a small percentage of the lift and horsepower. As a consequence the anti-torque or tail rotor is also driven, requiring consumption of power, and producing increased drag. Insofar as I have been able to determine means are not known for stopping or feathering the tail rotor when driving the pusher propeller, or, conversely, stopping or slowing the pusher propeller, when the tail rotor is in operation.
It can be seen that present dynamic drive system design for compound aircraft requires that the three propulsive devices, the main rotor, tail rotor, and pusher propeller, be rotated continuously during all flight modes. Having either the tail rotor or pusher propeller rotating when it is not required to produce a propulsive force reduces the overall operational efficiency through decreasing power available, increasing fuel consumption, producing unnecessary aerodynamic drag, and creating unnecessary wear (life cycles). A compound helicopter tail drive means is provided herein which will eliminate the major power losses associated with the kinematics of nonfunctional components during all flight modes, and low only operation of those components, when required during the various high speed, low speed, and hover modes of the aircraft.